Recently, TiN.sub.x has drawn attention as an important material for ultra large scale integrated circuits (ULSIC) having a line width of 0.5pm or less; a TiN.sub.x thin film is used as a diffusion barrier to protect shallow silicon junctions in the formation of Al contacts on a silicon wafer (J-Y Yun and S- W Rhee, Korean J. Chem. Eng., 13(5), 510(1996)).
Such diffusion barrier is required to have a low specific resistivity among others, and the specific resistivity of a TiN.sub.x film is influenced by the presence of impurities such as carbon and oxygen, structural defects such as vacancies, and also by the size of the grains that constitute the film. The grain size varies with the process conditions such as the deposition temperature, and in general, as the grain size becomes larger, the specific resistivity of the film becomes lower. It is particularly important to minimize impurity content of a TiN.sub.x film if it is intended for use as a diffusion barrier.
Chemical vapor deposition(CVD) has been widely used in the preparation of TiN.sub.x films, wherein inorganic compounds such as titanium tetrachloride (TiCl.sub.4) or organotitanium compounds such as tetrakis-dimethylamidotitanium (TDMAT) and tetrakis-diethylamidotitanium(TDEAT) are generally employed as precursors (I. J. Raaijmakers, Thin Solid Films, 247, 85(1994); S. R. Kurtz and R. G. Gordon, Thin Solid Films, 140, 277(1986))).
CVD of TiN.sub.x using TiC.sub.4 is disadvantageous in that the deposition temperature ranging from 500 to 600 C is generally too high for the ULSIC application, and also that chlorine may be incorporated in the final TiN.sub.x film, causing a corrosion problem (I. J. Raaijmakers, loco citato).
Further, CVD of TiN.sub.x using organotitanium compounds often entails problems such as carbon contamination, a low step coverage and high specific resistivity. For example, when a TiN.sub.x film is deposited by a CVD process using an organotitanium compound in the presence of gaseous ammonia as a reaction gas, ammonia reacts with the precursor in the gas phase to form particles. The particles thus formed get incorporated into the TiN.sub.x film and the smoothness of the film surface becomes deteriorated, leading to an increase in the specific resistivity (J. A. Prybyla et al., J. Electrochem. Soc., 140, 2695(1993)).
To avoid such problems, a thermal decomposition process which uses an organometallic compounds in the absence of ammonia has been suggested. However, this process gives a high level of contaminants and the resulting film has a high specific resistivity (I. J. Raaijmakers, loco citato).